Method of Creating and Utilizing Diversity in a Multiple Carrier Communication System

ABSTRACT

In many cellular systems, reusing spectrum bandwidth, creates problems in boundary regions between the cells and sectors where the signal strength received from adjacent base stations or adjacent sector transmissions of a single base station may be nearly equivalent. The invention creates a new type of diversity, referred to as multiple carrier diversity by utilizing multiple carriers, assigning different power levels to each carrier frequency at each base station, and/or offsetting sector antennas. The cell and/or sector coverage areas can be set so as to minimize or eliminate overlap between cell and/or sector boundary regions of different carrier frequencies. Mobile nodes traveling throughout the system can exploit multiple carrier diversity by detecting carriers and selecting to use a non-boundary carrier based on other system criteria in order to improve performance. Boundary carriers may, but need not be, identified and excluded from consideration for use by a wireless terminal.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of pending U.S. patentapplication Ser. No. 12/060,249 filed Mar. 31, 2008 and entitled “METHODOF CREATING AND UTILIZING DIVERSITY IN A MULTIPLE CARRIER COMMUNICATIONSSYSTEM” which is a divisional of granted U.S. Pat. No. 7,363,039 issuedon Apr. 22, 2008 and entitled “METHOD OF CREATING AND UTILIZINGDIVERSITY IN A MULTIPLE CARRIER COMMUNICATIONS SYSTEM” which claims thebenefit of U.S. Provisional Patent Application Ser. No. 60/401,920 filedon Aug. 8, 2002 and entitled “METHODS AND APPARATUS FOR IMPLEMENTINGMOBILE COMMUNICATIONS SYSTEM” and is a continuation-in-part of U.S. Pat.No. 6,788,963 issued on Sep. 7, 2004. Each of the preceding applicationsand patents are hereby expressly incorporated by reference in theirentirety.

FIELD OF THE INVENTION

The present invention is directed to wireless communications systemsand, more particularly, to methods and apparatus for improvingcommunication at the cell and/or sector boundaries of a multiple carrierspread spectrum system, by creating and utilizing carrier diversity asdescribed below.

BACKGROUND OF THE INVENTION

Wireless communications systems are frequently implemented as one ormore communications cells. Each cell normally includes a base stationwhich supports communications with end nodes, e.g., wireless terminalssuch as mobile nodes, that are located in, or enter, the communicationsrange of the cell's base station. Signals transmitted between a basestation and a mobile node may be transmitted in two possible directions,e.g., from the base station to the mobile node or from the mobile nodeto the base station. Transmission of signals from the base station tothe mobile is often called a downlink. In contrast, transmission fromthe mobile to the base station is commonly referred to as an uplink.Communication cells are subdivided into sectors in some systems. Withina cell or a sector of a cell, the unit of communications resource is asymbol, e.g., QPSK or QAM symbol. In the case of an orthogonal frequencydivision multiplexed (OFDM) system a symbol may be transmitted on afrequency tone (e.g., subcarrier frequency) for one time slot. The totalavailable communication resource, which tends to be limited, is dividedinto a number of such symbols (units) which can be used forcommunicating control and data information between a base station andone or more mobile nodes in the cell. For transmission purposes, thesubcarrier frequencies are modulated on a carrier frequency. The carrierfrequency and associated bandwidth encompassing the range of subcarrierfrequencies may be reused in sectors and cells.

Bandwidth reuse is an important method of improving spectral efficiencyin a cellular communication system. In particular, many cellular systemsuse special technology, such as spread spectrum technology, to allow thereuse of the same spectrum in multiple cells. This reuse of the samespectrum in multiple cells can result in operational problems at or nearthe cell boundaries. FIG. 1 shows a cellular system 100 utilizing thesame spectrum in adjacent cells. In FIG. 1, a first cell 106 representsan area of coverage 110 in which a first base station, base station 1102, may communicate with wireless terminals. A second cell 108represents an area of coverage 112 in which a second base station, basestation 2 104 may communicate with wireless terminals. Cells 106 and 108are neighboring cells which share a common boundary. In cellularcommunications systems, there are certain boundary areas where thesignal strengths, e.g., measured in terms of pilot power, received fromdifferent base stations are almost equally strong (sometimes referred toas 0 dB regions) and these areas are treated as the “boundary region” or‘boundary’ between cells. In FIG. 1, the coverage area 110 for basestation 1 102 and the coverage area 112 for adjacent base station 2 104overlap and create a boundary region 114.

Consider an exemplary case, in which a wireless terminal is located inthe cell boundary region 114. The wireless terminal can be fixed ormobile. When the wireless terminal is communicating with one of the basestations, e.g. base station 1 102, in the boundary region 114 theinterference from the other base station, e.g. base station 2 104, maybe almost as strong as the signal from the serving base station 102.Indeed, due to fading and other impairments in the wireless channel, thesignal may be much weaker than the interference from time to time.Therefore, the connection for that wireless terminal may not be robustin such a case. The signal reliability in the boundary region 114 may below and generally the power has to be boosted to overcome the noise. Aweak signal with low reliability may result in loss of or disruption ofcommunications for the user of the wireless terminal resulting incustomer dissatisfaction. Many wireless terminals are mobile devicesoperating on limited battery resources; therefore, any additionalexpenditure of power required by the mobile can be very significant, asit will directly reduce the user's operational time between batteryrecharge or replacement. In addition, the cost to serve that wirelessterminal in boundary region 114, in terms of power and bandwidthallocation in the serving base station 102, may be relatively high.Hence, there is a need for apparatus and methods to improve the servicein the cell boundary region 114.

Some cellular systems using special technology, such as spread spectrumtechnology, also subdivide the cells into sectors and allow the reuse ofthe same spectrum in all the sectors. This reuse of the same spectrum inall sectors of a cell can result in operational problems at or near thesector boundaries in addition to the above discussed cell boundaryproblems. The sector boundary region problems encountered are verysimilar or identical to the cell boundary regions problems. Hence, thereis also a need for apparatus and methods to improve the service in thesector boundary regions.

SUMMARY OF THE INVENTION

As discussed above, in many cellular systems, reusing spectrumbandwidth, creates problems in boundary regions between the cells andsectors where the signal strength received from adjacent base stationsor from adjacent sector transmissions of a single base station may benearly the same. In those regions, the intercell and/or intersectorinterference levels are relatively high which can lead to lowreliability and poor quality of service. The invention uses apparatusand methods to create different cell and/or sector boundary regions fordifferent carrier frequencies transmitted from a base station whichsupports the use of multiple carrier frequencies. By engineering theoverall system to take advantage of the fact that different boundaryregions are associated with different carrier frequencies and bysupporting intracell and/or intercarrier handoff wireless terminals canbe assigned to carriers in a way that minimizes or reduces the effect ofboundary interference.

The invention creates a new type of diversity, referred to as multiplecarrier diversity with respect to cells and/or sectors by utilizingmultiple carriers. In accordance with one feature of the invention, thisis accomplished, in part, by assigning different power levels todifferent carrier frequencies at a base station. In some exemplaryembodiments there is at least a difference in power levels of 20%between two carriers used by the same base station. In other embodimentsthe difference in power levels may be greater or lower. For example,power differences of at least 10, 30, 40 and 50 percent may be and areused in various other embodiments.

This summary and various parts of the application describe multiplecarrier diversity with respect to two exemplary carrier frequencies Aand B; however, the principles are applicable to other numbers ofcarrier frequencies, and the invention is not limited to the exemplarytwo carrier embodiment. For example, 3 or more carriers may be used at abase station corresponding to a cell. In one particular exemplaryembodiment, each base station transmits two carrier frequency signalseach having a different carrier frequency A and B, respectively, andeach carrier signal having communications bandwidth. The use ofdisparate power levels creates coverage areas for each carrier signal.This results in different intercell boundaries for different carriersignals transmitted by the base station. In a multiple base stationsystem, power levels can be adjusted and controlled at a first basestation so that the carrier frequency A boundary and the carrierfrequency B boundary created as a result of an adjacent base stationusing the same carrier frequencies but different power levels aresufficiently separated to create small or no overlap in the boundaryregions. The power levels for the carrier signals can be, and in someembodiments, are, chosen at the second base station so that the carrierfrequency A boundary and the carrier frequency B boundary will notoverlap. In one particular exemplary embodiment the difference incarrier signal power levels with regard to signals generated by a basestation is at least 20%.

In various embodiments, there is a relationship between the power levelsof carrier signals of adjacent base stations. For example, in someembodiments, if P_(A1)>P_(B1), then P_(A2)<P_(B2); if P_(A1)<P_(B1),then P_(A2)>P_(B2). In some embodiments P_(A1)=P_(B2)<P_(A2)=P_(B1).

In some implementations the power levels are chosen, in accordance withone or more of the above carrier signal power relationships, to insurethat there is less than a 50% overlap between a carrier frequency A cellboundary region and a carrier frequency B cell boundary region. In someimplementations, there is no overlap between the frequency A and B cellboundary regions.

The methods and apparatus of the invention can also be used to createcarrier diversity with respect to sectors, in a sectorized environment,e.g., where a single cell includes multiple sectors into which the basestation transmits by using multiple antennas or antenna elements, e.g.,one per sector per carrier frequency, or other techniques such asmultiple antennas in combination with beam forming.

By using antennas which are offset from one another for each of multiplecarriers, or other techniques to form different sector coverage areasfor the sectors corresponding to different carrier frequencies, it ispossible to create a coverage region where the sector boundaries fordifferent carrier frequencies used within a cell will be different. Bycontrolling the sector locations for different carrier frequencies, itis possible to create a cell where there is little or no overlap betweenboundary areas corresponding to sectors corresponding to differentcarrier frequencies. In such a case, when a wireless terminal is in aboundary area corresponding to one sector it is possible to switch toanother carrier frequency in the cell, e.g., as part of an intercarrierhandoff. Since sectors corresponding to different carriers aredifferent, in the sectorized embodiment, an intracell intercarrierhandoff is normally an intracell intersector handoff. As a wirelessterminal moves in a cell, or because of different loading conditions,multiple intracell intercarrier handoffs may occur even in the casewhere a carrier being used to support a communication session has notdegraded to the point where such a handoff is necessarily required froma communications perspective. While such handoffs may complicateprocessing slightly, by switching between the carriers at a particulartime, power efficiency and increased overall data throughput can beachieved since the effect of intersector interference can be minimized.

In embodiments where different antennas are used for each sector of acell, with different antennas being used for each carrier frequency, theantennas used to transmit different carrier frequencies are offset fromeach other to provide different coverage areas and different sectorboundaries for each carrier frequency. In one such embodiment, theantennas corresponding to a first carrier frequency are offset at least30 degrees from an antenna corresponding to a second carrier frequency.In other embodiments other offsets are used, e.g., offsets of at least10, 20, 30, 40, or 50 degrees are used. In one particular embodimentwhere a cell is divided into three sectors and at least two carrierfrequencies are used, the antennas corresponding to a first carrierfrequency are offset from antennas corresponding to a second carrierfrequency by at least 60 degrees.

In addition to methods and apparatus for creating multicarrier diversitythe present invention is directed to method and apparatus for exploitingthe benefits made possible from such diversity. To take advantage ofsuch diversity, intracell carrier handoffs are implemented in caseswhere a carrier handoff is not necessarily necessitated by the inabilityto successful continue communicating using a carrier being used for acommunications session but because communications and thus systemefficiencies can be obtained by having a wireless terminal switchcarriers. Thus, intra-cell and inter-sector inter-carrier handoffs mayoccur while the SNR of the carrier remains reasonably good, e.g.,between 3 dB and 0 dB. Dropping below an upper predetermined threshold,e.g., a 3 dB threshold, may trigger consideration of an intercarrierhandoff. The decreasing SNR combined with an SNR below 3 dB may indicateentry into a boundary region. Before a intercarrier handoff occurs whilethe SNR remains satisfactory, e.g., above 0 dB, the system may, and insome embodiments does, require that the SNR to remain below the upperthreshold, e.g., 3 dB, for some predetermined period of time, beforeimplementing an intra-cell intercarrier handoff. For example the SNR maybe required to stay below 3 dB for 1 seconds, to reduce the potentialthat short noise bursts might cause an intercarrier handoff. Otherperiods of time are possible.

Wireless terminals traveling throughout the sectors and cells of thesystem can exploit the multiple carrier diversity by detecting carriersignal conditions and selecting, or having the base station select,carriers to be used at any given time to avoid the use of a particularcarrier while in a cell or sector boundary corresponding to saidcarrier. The carrier selection process can be performed as a function ofother information as well, e.g., carrier loading condition information,to provide an efficient carrier allocation scheme.

As discussed above base stations in accordance with the invention cantransmit at multiple carrier frequencies. In some embodiments operatingin a sectorized environment, the base stations may have sectortransmitter circuitry for each sector and multi-sector antennas. In thecase of multi-sector antennas, at least one transmitting elementnormally exists for each carrier frequency, with the transmittingelements of different carriers being offset to create different boundaryregions for different carrier signals. The base stations have powermanagement routines for controlling and maintaining the transmissionpower for each the carrier frequencies and setting power leveldifferences between carrier frequencies at a given base station.

In some embodiments, the base station can determine from information,e.g., channel condition feedback information, received from the wirelessterminal whether a wireless terminal is in a cell boundary area of aspecific carrier frequency, whether the wireless terminal is in a sectorboundary area of a specific carrier frequency, or whether the wirelessterminal in a non-sector boundary area of a specific carrier. A wirelessterminal can, and in various implementations does, determine if it is ina sector or cell boundary by comparing the power level of pilotscorresponding to the same carrier frequency but received from differentcells or sectors. The receipt of pilots corresponding to the samecarrier frequency from different transmitters having the same orapproximately the same power level, indicates that the wireless terminalis in a boundary region for the particular carrier frequency. Based onthe measurement information, the wireless terminal may send anintercarrier handoff request to the base station. Based on theinformation, received from the wireless terminal and/or information thebase station has detected or recorded, the base station can with itsscheduler, inter-carrier handoff routine, and sector management routineallocate, when possible, the wireless terminal to a carrier which doesnot have a boundary region corresponding to the wireless terminal'scurrent location. If multiple non-boundary carriers are available, thebase station or wireless terminal will normally select a carrier whichdoes not have a boundary area at the wireless terminal's currentlocation based upon other additional factors such as traffic loading,power considerations, noise levels, etc.

The base station and/or wireless terminal can initiate and performintra-cell inter-carrier handoffs for a wireless terminal based on celland/or sector boundary region information, intercell channelinterference information, intersector channel interference measurements,signal strength degradation, or other considerations. The intra-cellhandoff performed by the base station may be initiated by a signal orrequest from the wireless terminal. The intra-cell handoff need not beforced by an event such as loss of carrier signal, but may be due to aproactive monitoring of available carriers by the wireless terminal, anda system decision to change carrier frequencies for some other reasonsuch as load balancing or the anticipation of the entrance into acell/sector boundary region, e.g., a region where intercell interferenceor intersector interferce causes the SNR for a carrier to be below 3 dBor some other value, e.g., 6, 5, 4, 2, 1 dB in some embodiments. As aresult, a mobile may be involved in multiple intercarrier handoffs whileremaining in a cell and without actually loosing the ability tocommunicate with a base station using a carrier to which it is assignedat any particular time. In fact, in some cases, 5 or more intracellintercarrier handoffs occur during a single communications sessionbetween a wireless terminal and a base station without the wirelessterminal ever suffering sufficient carrier signal interference tonecessitate changing of carriers to maintain reliable communication withthe basestation.

In various embodiments, the wireless terminal, in accordance with thepresent invention, proactively and repeatedly monitors for carriertransmissions signal strength from base station transmissions of thevarious cells and sectors and switches carriers used to communicate witha base station based on the signal strength information despite signalquality on a carrier being used being sufficient (e.g., SNR beinggreater than 0 dB) to support continued communication at a communicationrate which is being supported at, and before, the time the decision toswitch carriers has to be made due to signal loss. In variousembodiments the wireless terminal has multiple analog receiver chains,e.g., filter and demodulator chains, in which case one receiver remainson the carrier frequency used to communicate with the base station,while the other receiver chain is used to monitor for alternate carrierswhich may be used to communicate with the same base station.Alternately, the wireless terminal may include a single receiver chainincluding a channel filter and demodulator and therefore may be limitedto receiving one carrier signal at a time. In this case, the wirelessterminal may temporarily use its receiver to monitor for other carrierswhen not processing the carrier currently being used to communicate witha base station. In accordance with one embodiment of the presentinvention, the analog filter is adjustable and programmable with theability to lock onto a particular selected carrier. This single receiverchain implementation is particularly possible and cost effective in awireless data terminal, where the terminal can use the period of timeduring which no reception is needed from the current serving carrier tomonitor other carriers, e.g., such as during a portion of a sleep orhold state of operation. As discussed above, in various embodiments thewireless terminal can do one or more of the following: measure one ormore of the following: carrier signal strength, intercell channelinterference, intersector channel interference; can differentiatebetween different types of interference, identify and classify carriersas cell and/or sector boundary carriers, form a list of candidatecarriers, that are carriers received of acceptable strength and qualityexcluding the identified boundary carriers, and then select a carrier touse. The wireless terminal may feed back some or all of the informationcollected to the base station. The wireless terminal may make aselection of the carrier to use may be based upon other considerationsother than boundary interference levels such as traffic loading, powerconsideration, or user priority. The selection of the carrier can resultin the wireless terminal signaling a base station to initiate aninter-carrier handoff. The inter-carrier handoff may be an intra-cellintercarrier handoff resulting in a change in carrier frequencies and/orsectors with a cell at a single base station or may be an inter-cellinter-carrier handoff between different cells with different basestation.

Numerous additional features and benefits of the present invention arediscussed in the detailed description which follows:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates two adjacent base stations with an overlapping cellboundary region.

FIG. 2A illustrates two adjacent base stations operating on a firstcarrier frequency (A) but with different power levels for each basestation implemented in accordance with the present invention.

FIG. 2B illustrates the two adjacent base stations of FIG. 2A operatingon a second carrier frequency (B) but with different power levels foreach base station implemented in accordance with the present invention.

FIG. 2C shows the two adjacent base stations of FIG. 2A operatingsimultaneously on both carrier frequencies (a combination of FIG. 2A andFIG. 2B) illustrating that the power levels may be chosen such that thecarrier frequency A cell boundary does not overlap the carrier frequencyB cell boundary and illustrating multiple carrier diversity with regardto cells in accordance with the present invention.

FIG. 2D illustrates a more realistic representation of FIG. 2Cillustrating that the frequency A and B cell boundaries will actually becell boundary regions, but that the power levels can be controlled as inFIG. 2C so that the boundary regions do not overlap in accordance withthe invention.

FIG. 3 illustrates a flow chart whose method may be implemented toexploit multiple carrier diversity with regard to cell boundaries inaccordance with the invention.

FIG. 4A illustrates a cell, subdivided into three sectors, surrounding abase station utilizing a sectorized antenna and operating on a firstcarrier frequency A.

FIG. 4B illustrates the base station of FIG. 4A with a second sectorizedantenna operating on a second carrier frequency B. The antenna of Fib4B, has been offset 60 deg with respect to the antenna of FIG. 4Aresulting in the three sectors of the cell of FIG. 4 b being offset by60 deg with respect to the three sectors of FIG. 4A.

FIG. 4C is an overlay of FIGS. 4A and 4B showing the base station ofFIG. 4A, simultaneously transmitting on the two carrier frequencies Aand B and further illustrating that if the antennas are offsetsufficiently, the sector boundary regions will not overlap. Theimplementation of FIG. 4 c thus creates multiple carrier diversity withregard to sectors in accordance with the invention.

FIG. 4D illustrates a more realistic representation of FIG. 4Cillustrating that the frequency A and B sector boundaries will actuallybe sector boundary regions, but that the antenna offset can be chosen asin FIG. 4C so that the boundary regions do not overlap in accordancewith the invention.

FIG. 5 illustrates a flow chart whose method may be implemented toexploit multiple carrier diversity with regard to sector boundaries inaccordance with the invention.

FIG. 6 illustrates an example of an inter-carrier handoff at a singlebase station using multiple carriers of different power levels inaccordance with the present invention.

FIG. 7 illustrates an exemplary communications system implementingmultiple carrier diversity across cells and sectors in accordance withthe present invention.

FIG. 8 illustrates an exemplary base station implemented in accordancewith the present invention.

FIG. 9 illustrates an exemplary end node (wireless terminal, e.g.,mobile node) implemented in accordance with the present invention.

DETAILED DESCRIPTION

When a large amount of bandwidth is allocated to a cellular system, thebandwidth is often divided into two or more portions, each of which hasa distinct carrier deployed. The spectrum assigned to each distinctcarrier deployed may or may not be adjacent. These deployments arecalled multiple carrier systems. In a multiple carrier system, deployinga spread spectrum technology, the bandwidth associated with each carriermay be reused in all cells.

The current invention is directed to methods and apparatus for improvingthe service at the cell boundaries and sector boundaries of a multiplecarrier spread spectrum system, by creating and utilizing ‘multiplecarrier diversity’ as described below.

Normally, in a communications system, the system is engineered for onecarrier. If a second carrier is added, to be used by the same basestation, typically, the same design parameters, e.g. power requirements,etc., are used resulting in the same coverage area for both carriers. Insuch a case, the two carriers will have the same general cell boundaryand same cell boundary areas will occur between adjacent cells. Inaccordance, with a novel feature of the invention, the power between themultiple carriers is varied in a controlled and engineered manner,resulting in different cell boundaries of selected sizes for eachcarrier of a cell. In addition, the power applied by adjacent basestations with respect to each carrier frequency may be varied usingsimilar reasoning in the multiple adjacent cells of the communicationssystem. This creates different and potentially non-overlapping boundaryareas for each carrier used in a cell.

FIGS. 2A, 2B, 2C, and 2D are used to illustrate a method of creatingmultiple carrier diversity, in accordance with the invention. In FIG.2A, an exemplary system 200 includes a first base station, base station1 (BS1) 202 and a second base station, base station 2 (BS2) 204. BS1 202has a nominal transmission power of a carrier with frequency A, P_(A1),as represented by arrow 203, with a cellular coverage area enclosed bysolid line circle 206. BS2 204 has a nominal transmission power of acarrier with frequency A, P_(A2), as represented by arrow 205, with acellular coverage area enclosed by solid line circle 208. Cellularcoverage area 206, as shown, may be smaller than cellular coverage area208 due to a lower level of transmission power of BS1 202 with respectto BS2 204 (P_(A1) 203<P_(A2) 205). FIG. 2A also includes a carrierfrequency A boundary area 210 between adjacent cell areas 206 and 208.In FIG. 2B, an exemplary system 220 includes the two base stations, BS1202 and BS2 204, each with a nominal transmission power of a carrierwith frequency B, P_(B,1) as indicated by arrow 223, P_(B,2) asindicated by arrow 225, respectively, and each with a carrier frequencyB cellular coverage area enclosed by dashed line circles 226, 228,respectively. Coverage area 226, as shown, is larger than coverage area228, and may be due to a higher level of power applied at BS1 202 thanat BS2 204 with respect to carrier with frequency B (P_(B1) 223>P_(B2)225). FIG. 2B also includes a carrier frequency B boundary area 230between cell areas 226 and 228.

In the exemplary system 240 of FIG. 2C (a combination of FIGS. 2A and2B), both base stations 202, 204 are shown to use both carriers A and Bsimultaneously. In accordance with the invention, the transmissionpowers P_(A,1),P_(B,1) (203, 223) of the two carriers, A and B,respectively, in BS1 202 are selected to be different. Similarly, inaccordance with the invention, the transmission powers P_(A,2),P_(B,2)(205, 225) of the two carriers, A and B, respectively, in BS2 204 areselected to be different. FIG. 2C shows that in BS1 202, the differencein the nominal transmission power of carrier A, P_(A,1), 203 from thatof carrier B is P_(B,1).223, results in different size cellular coverageareas 206, 226, respectively for BS1 202. Similarly, in base station 2204, the difference in the nominal transmission power of carrier A,P_(A,2), 205 from carrier B, P_(B,2)., 225 results in different sizecellular coverage areas 208, 228, respectively for BS2 204. The cellboundary between adjacent cell for carrier A 210 is determined byP_(A,1) 203 and P_(A,2),205 and the cell boundary between adjacent cellsfor carrier B 230 is determined by P_(B,1) 223 and P_(B,2) 225. Theexample of FIG. 2A, 2B, 2C shows an ideal case where the transmissionpowers have been matched precisely so that the A frequency boundary 210is a single point and the B frequency boundary 230 is a single point. Inactual operation, each carrier frequency boundary may be represented bya region of overlapping coverage between the two base stations 202, 204.In some embodiments each carrier frequency boundary region may bedefined as an area where the difference in carrier signal strength, fora particular carrier frequency, from the base stations 202,204 is 3 dBor less. In other embodiments, a carrier frequency cell boundary regionmay be defined in terms of a signal difference of e.g., less than 1, 2,4, 5 or 6 dB depending on the implementation. In still otherembodiments, different levels of interference may define the carrierfrequency cell boundary region. The exemplary system 260 of FIG. 2Dillustrates a frequency A boundary region 270 and a frequency B boundaryregion 280. In accordance with the invention, the values of thosetransmission powers, P_(A,1) 203, P_(A,2) 205, P_(B,1) 223, and P_(B,2)225 can chosen to be sufficiently different such that the cell boundaryregions of carriers A and B (270, 280) are sufficiently non-overlapping.In one embodiment, P_(A,1)=P_(B,2)<P_(B,1)=P_(A,2). FIGS. 2C and 2D showexamples of cell boundaries of carriers A and B, that are apparentlynon-overlapping, in accordance with the invention.

In accordance with the present invention, different carriers have beenutilized via a design approach to structure the system withnon-overlapping cell boundaries, e.g. boundary region for carrier A 270and boundary region for carrier B 280 of FIG. 2D are non-overlapping,creating a new type of diversity, namely ‘multiple carrier diversity’.In particular, if a wireless terminal is located in the cell boundaryregion of one carrier, it is likely not in the cell boundary region ofother carriers. Therefore, a wireless terminal may measure the signalquality of all the carriers and select to use a proper carrier such thatthe wireless terminal is not in the cell boundary region of the selectedcarrier. For example, with respect to FIG. 2D, if a wireless terminal isin the cell boundary region for carrier frequency A 270, it is not inthe cell boundary region for carrier frequency B 280, and thereforeshould choose to operate on carrier frequency B. The idea of utilizingmultiple carrier diversity is illustrated further in the flow chart ofFIG. 3. Note that in the example a wireless terminal monitors signalinterference associated with each of the carriers in a cell andproactively switches between them to select a carrier that is not aboundary carrier without necessarily losing contact with the basestation on an existing carrier that is being used. For example, carrierswitching may occur when an SNR decreases to between 6 and 0 dB andanother carrier is available. In some embodiments, the carrier selectionprocess, includes selecting the non-boundary carrier, with the leastinterference or the least traffic loading.

FIG. 3 illustrates a flow chart 300 whose method may be implemented by awireless terminal to exploit the multiple carrier diversity, inaccordance with the invention. The process starts with step 301 where awireless terminal is powered and able to receive base station signals.Operation proceeds to step 302, where the wireless terminal measures thesignal strengths; e.g. via pilot tone reception measurements, of theadjacent base stations and the serving base station in all, or a subsetof all, the carriers used by the base stations. There are several waysin which this can be achieved. One such receiver architecture has twoRadio Frequency (RF) and baseband receiver chains. Each receiver chainincludes a carrier filter and a demodulator arranged in series. Each ofthe receivers is capable of receiving one carrier with the filters ofeach chain being designed to pass the carrier frequency associated withthe receiver chain while rejecting other carrier frequencies. Thus whileone receiver is tuned to a particular carrier the other receiver chaincan be used to search for alternative carriers. In accordance with thepresent invention, the analog filter(s) used in each filter chain isadjustable and/or programmable in order to be able to lock onto aparticular selected carrier. This first approach uses two separateprocessing chains, e.g., relatively expensive analog processing chains.Normally, a mobile terminal doesn't talk to two carriers simultaneouslybecause two separate analog processing chains would be required,resulting in an expensive implementation. Thus, for cost reasons, inmany cases a single receiver chain is used.

If only one receiver chain is available, the wireless terminal maytemporarily use its receiver to monitor other carriers when notprocessing the carrier currently being used to communicate with a basestation. In accordance with one embodiment of the present invention, theanalog filter is adjustable and programmable with the ability to lockonto a particular selected carrier. This is particularly possible andcost effective in a wireless data terminal, where the terminal can usethe period of time during which no reception is needed from the currentserving carrier to monitor other carriers.

A third alternative is to have a single radio frequency receiver capableof receiving signals encompassing several carriers and have base bandreceivers which can tune to the different carriers and measure theirsignal strengths. The analog filtering required for this method may alsobe adjustable and programmable.

Using the signal strength measurements of step 302, the wirelessterminal in step 304 identifies whether it is located in the cellboundary region of any carriers. Those carriers, identified by thewireless terminal to be in a cell boundary region, are called andclassified as cell boundary carriers. This may be done by comparingpilot tone strengths and determining if pilot tones from different cellshave at least a 30% difference in received power with lower powerdifferences being interpreted, in some embodiments as an indication of adevice's presence in a cell boundary region. In other embodiments apower difference of 20% or less between carrier signals from differentcells is used to define boundary regions. Alternatively, an SNR of 3 dBor less may be used as indicative of being in a cell boundary region. Insome cases, the carrier selection process is performed when a decreasein the SNR is detected on the carrier being used and the SNR drops belowan upper threshold, e.g., 3 dB, but still remains acceptable, e.g.,above 0 dB.

Operation proceeds from step 302 to step 306, where the wirelessterminal determines candidate carriers. The candidate carriers are thosecarriers which have been received in step 302 and deemed to be ofacceptable signal strength but excluding the cell boundary carriersidentified in step 304, e.g., carriers suffering from intercellinterference of at least 30%. Next, in step 308, the wireless terminaldetermines which carrier to be used, e.g. as a function of signalinterference, signal quality and/or other factors. In accordance withthe invention, the wireless terminal selects from amongst the candidatecarriers, if possible, and does not, if possible, select a cell boundarycarrier. Among the candidate carriers, the wireless terminal may in step308 select a carrier as a function of other conditions or concerns suchas traffic loading. Traffic loading information may be obtained, e.g.,based on information available to the node such as monitored channelassignment information. The above carrier monitoring and selectionprocedure may repeat periodically and/or frequently as implemented withoperation returning to step 302.

FIGS. 4A, 4B, 4C, and 4D are used to illustrate another method ofcreating multiple carrier diversity, in accordance with the invention,in a sectorized environment. In this illustrated example, a base station402 uses three-sector antennas. FIG. 4A illustrates an exemplary threesector cell 400 surrounding base station 402 with a coverage area 404for a carrier of frequency A. The coverage area 404 is broken into threesectors: S_(1A) 406, S_(2A) 408, and S_(3A) 410 with sector boundaries:sector boundary 1-2 412, sector boundary 2-3 414, and sector boundary3-1 416. FIG. 4B illustrates an exemplary three sector cell 420surrounding base station 402 with coverage area 404 for a carrier offrequency B. The coverage area 404 is broken into three sectors: S_(1B)426, S_(2B) 428, and S_(3B) 430 with sector boundaries: sector boundary1-2 432, sector boundary 2-3 434, and sector boundary 3-1 436. FIG. 4Cillustrates in cell 440 (as an overlay of FIGS. 4A and 4B, an exemplarycase where base station 402 has two carriers, of frequency A and B,which are to be used in all the sectors simultaneously. The totalcoverage area 404 may be the same for both frequencies A and B.Normally, in sectorized cells using multiple frequencies, the antennasused for each frequency are aligned so that the sectors and sectorboundaries are the same for each frequency. In accordance with theinvention, the two three-sector antennas are placed such that thesectorization orientations of the two carriers at the base station 402are sufficiently offset, e.g. 60 deg, as illustrated in FIG. 4C toprovide ‘multiple carrier diversity’. The example of FIG. 4C shows anoffset of approximately 60 deg between the sector boundaries of the twocarrier frequencies: (412,432), (414,434), (416,436). The sectorboundaries 412, 414, 416, 432, 434, 436, i.e., boundaries betweenadjacent sectors, are shown in FIG. 4C for an idealized system. Inactual operation, sector boundary areas will exist between the sectorsfor each frequency. In some embodiments each carrier frequency sectorboundary region is defined as an area where the difference in carriersignal strength from the adjacent sector base station transmissions fora given carrier frequency is 3 dB or less. In other embodiments,different levels of interference may define the sector carrier frequencysector boundary region, e.g., differences of 2 dB may be used in someembodiments or other values such as 1 or 4 dB. FIG. 4D, illustrates anexemplary cell 460 with cellular coverage area 404 for base station 402employing a 3 sector implementation with 60 deg offsets, andsimultaneous dual carrier frequency operation. FIG. 4D includesfrequency A sector boundary areas: boundary region 1-2 462, boundaryregion 2-3 464, and boundary region 3-1 466. The frequency A sectorboundary regions 462, 464, 466 may be identified by crosshatch shadingin FIG. 4D. FIG. 4D also includes frequency B sector boundary areas:boundary region 1-2 472, boundary region 2-3 474, and boundary region3-1 476. The frequency B sector boundary regions 472, 474, 476 may beidentified by diagonal line shading in FIG. 4D. The sector boundaryregions of frequency A carriers 462, 464, 466 and the sector boundaryregions of frequency B carriers 472, 474, and 476 are sufficientlynon-overlapping, thereby creating multiple carrier diversity with regardto sectorization, similar to that shown in FIG. 2D with regard to cells.Wireless terminals in such a sectorized system may apply the same logicshown in FIG. 3 for cells, to sectors to utilize the advantages ofmultiple carrier diversity, in accordance with the present invention.Specifically, wireless terminals operating in an A carrier frequencyboundary region 462, 464, 466 should and will choose to operate oncarrier frequency B; while wireless terminals operating in a B carrierfrequency boundary region 472, 474, 476 should and will choose tooperate on carrier frequency A. Wireless terminals outside, the sectorboundary regions 462, 464, 466, 472, 474, 476, yet still inside thecellular coverage area 404, may choose to operate on either carrierfrequency A or B depending on other constraints such as loading andsignal interference due to conditions other than inter-cell orinter-sector interference.

FIG. 5 illustrates a flow chart 500 whose method may be implemented by awireless terminal to exploit multiple carrier diversity with regard tosectorization, in accordance with the invention. The process starts withstep 501 where a wireless terminal is powered on and capable ofreceiving base station signal. Operation proceeds to step 502 where thewireless terminal measures the signal strengths of the available sectortransmissions (e.g. pilots) from the serving base station for all or asubset of all the carriers. There are several ways in which this can beachieved, depending on receiver design, as previously described indetail with respect to FIG. 3.

Next, in one embodiment, using the signal strength measurements of step502, the wireless terminal in step 504 identifies whether it is locatedin the sector boundary region of any carriers. Those carriers,identified by the wireless terminal to be in a sector boundary region,are called and classified as sector boundary carriers. Thisdetermination may be based on signal interference levels. Operationproceeds to step 506, where the wireless terminal determines candidatecarriers. The candidate carriers are those carriers which have beenreceived in step 502 and deemed to be of acceptable signal strengthexcluding the sector boundary carriers identified in step 504. Next, instep 508, the wireless terminal determines which carrier to be used. Inaccordance with the invention, the wireless terminal selects fromamongst the candidate carriers, if possible, and does not, if possible,select any sector boundary carrier. Among the candidate carriers, thewireless terminal may in step 508 select the carrier as a function ofone or more conditions or concerns such as traffic loading. The abovecarrier monitoring and selection procedure may repeat periodicallyand/or frequently as implemented with operation returning to step 502.The carrier selection process occurs even when the existing carrierremains suitable for use, e.g., has an SNR of above 0 dB. In someembodiments, the carrier signal is deemed unsuitable for use when theinterference signal has a power level 80% of the power level of a signalof interest. To reduce the number of handoffs, the selection process maybe limited to cases where a decrease in SNR below a threshed level isdetected as will occur upon entry into a boundary area. The handoff mayfurther be restricted by requiring the decrease in SNR to be maintainedfor some predetermined period of time. Drops below a second threshold,e.g., 1 dB may trigger an immediate handoff. Given the describedintercarrier handoff process, the wireless terminal may switchrepeatedly between carriers while communicating with the same basestation even though an existing carrier remains acceptable, e.g., above0 dB in terms of SNR.

Alternately, in another embodiment, operation proceeds from step 502 tostep 509, in which the wireless terminals feeds back information to theserving base station including, e.g. signal strength/quality of thereceived sector transmissions from the serving base station for thecarriers. Proceeding to step 510, the serving base station identifiesthe sector boundary carriers for each wireless terminal. Next, in step511, the serving base station determines candidate carriers for eachwireless terminal, which are the received carriers of step 502 ofacceptable strength excluding the sector boundary carriers identified instep 510. Next, in step 512, the serving base station determines whichcarrier to be used for each wireless terminal. In accordance with theinvention, the serving base station selects for each wireless terminalfrom amongst the specific wireless terminal's candidate carriers, ifpossible, and does not, if possible, select a sector boundary carrier.Among the candidate carriers, the serving base station may for eachwireless terminal in step 512 select the carrier according to variousconditions or concerns such as traffic loading. The above carriermonitoring and selection procedure may repeat periodically and/orfrequently as implemented with operation returning to step 502.

The same methods of creating multiple carrier diversity can be used in abeam-forming multiple antenna system, where a different set of antennacoefficients are used for different carriers to create different carriertransmission patterns. In such a case, in accordance with the invention,the boundary areas of different carriers are generated to avoidoverlapping boundary areas.

The two methods shown in FIGS. 2A,2B,2C,2D and FIGS. 4A,4B,4C,4D can becombined to minimize or eliminate overlapping boundary areascorresponding to different carriers.

Although shown for exemplary cases of 2 cells in FIGS. 2A,2B,2C,2D and 3sectors in FIG. 4A,4B,4C,4D, the concepts are equally applicable and maybe extended to other implementation involving other numbers of cellsand/or cells with other numbers of sectors.

The two methods of creating multiple carrier diversity shown in FIGS.2A,2B,2C,2D and FIGS. 4A,4B,4C,4D can be generalized in accordance withthe invention. In general, the boundary regions (e.g., cell boundary, orsector boundary) are determined in large part by some system parametersused by the base stations, such as the transmitted power of basestations or offsets between sectorized antennas. In accordance with theinvention, these system parameters are selected in the system design andpurposely made different for different carriers such that the boundaryregions of individual carriers have minimum or even zero overlap. Thewireless terminals moving throughout the system may exploit the multiplecarrier diversity that has been established by, identifying andexcluding any boundary carriers, and then selecting to operate onanother available non-boundary carrier, which has been made available bythe multiple carrier diversity.

FIG. 6 600 illustrates examples of inter-carrier handoffs at a singlebase station 602 using multiple carriers of different power levels inaccordance with the present invention. A cell boundary 604 for carrierfrequency A represents the coverage area in which an exemplary wirelessterminal 608 may communicate using carrier frequency A with base station602 under ideal conditions, while a cell boundary 606 for carrierfrequency B represents the coverage area that the exemplary wirelessterminal 608 may communicate using carrier frequency B with base station602 under ideal conditions. Cell boundary 612 for carrier frequency Arepresents the coverage area in which exemplary wireless terminal 608may communicate using carrier frequency A with an adjacent base station,while a cell boundary 614 represents the coverage area in which theexemplary wireless terminal 608 may communicate using carrier frequencyB with the adjacent base station. Area 616, shown with line shadingascending from left to right, represents a carrier frequency A boundaryregion between adjacent cells. Area 618, shown with line shadingdescending from left to right, represents a carrier frequency B boundaryregion between adjacent cells. A dashed line arrow 610 representswireless terminal, e.g. mobile node, 608 crossing carrier frequency Acell boundary 604. A solid line arrow 620 represents wireless terminal,e.g., mobile node 608 crossing into a carrier frequency A boundaryregion between adjacent cells 616.

Typically, in previous wireless systems with a single base station usingmultiple carrier frequencies, the transmission power level would besubstantially equivalent for the multiple carriers used, resulting inthe same cell boundaries for all carrier frequencies. As a wirelessterminal moved throughout the cell, it would lock onto one frequency andremain on that one frequency while communicating with that base stationuntil a hand-off occurs to another adjacent cell with a new base stationor until reception is lost due to some variation such as a change ofnatural conditions, e.g., physical obstructions, weather conditions,etc. In accordance with the invention, the wireless terminal proactivelymonitors and searches for alternative carriers and performsinter-carrier hand-offs using the same base stations as part of normaloperation resulting in better traffic load balancing, increasedefficiency, and an improvement in system performance. In one example,wireless terminal 608, located within the cell boundaries for bothfrequencies A and B (604,606), respectively, may have locked onto aspecific carrier, e.g. the stronger carrier frequency, B; however, thewireless terminal 608 may decide to move to another carrier, e.g.carrier frequency A, for load balancing purposes, and thus perform aninter-carrier handoff at base station 602. This inter-carrier hand-offwould free up the higher power carrier frequency for use by anotherwireless terminal at a different location that may require increasedsignal strength to continue operation. In another example, the wirelessterminal 608 may be operating on the weaker carrier signal, e.g. carrierA, but may have detected by monitoring that the loading on carrier B,the stronger carrier signal, is light enough that it may and then doestransition to carrier B; this may result in the power expenditure ofwireless terminal 608 being reduced, an important consideration forwireless devices operating on limited battery resources. In anotherexample, wireless terminal 608 may lose the one carrier, e.g. carrierfrequency A, that it is using as it crosses cell boundary for carrierfrequency A 604, as illustrated by dashed arrow 610. The cell boundaryfor carrier frequency A 604 is actually an intracell carrier frequency Aboundary region. In some embodiments this intracell boundary region maybe defined as an area where the difference in carrier signal strengthfrom the base station falls off to approximately 0 dB. In otherembodiments, different levels of signal strength may define theintracell carrier frequency cell boundary regions, e.g., SNR levels of1, 2, 3, 4, 5 or 6 dB or less may, and sometimes are, used to definecell boundary regions. A switch to carrier frequency B, from carrier A,is used to maintain or to reestablish communications. In accordance withthe invention, the wireless terminal, e.g. mobile node 608 proactivelymonitors and searches for alternative carriers, collects data on thecarriers, makes decisions on which carrier to be used within the celland/or feeds back information to the base station 602 to decide whichcarrier to use with the cell. This allows the system 600 to anticipatethe necessity of inter-carrier handoffs at the single base station 602and efficiently perform the inter-carrier handoff operations before lossof communication occurs or with minimal disruption of communicationsbetween the base station 602 and the mobile 608.

In another example, of an intracell intercarrier handoff, wirelessterminal 608 is located within the cell boundaries for both carrierfrequency A and B, (604,606), respectively, and is operating on carrierfrequency A to communicate with base station 602. Wireless terminal 608moves and crosses into the boundary region between adjacent cells forcarrier frequency A 616. The wireless terminal 608 has been proactivelysearching for candidate carriers. The wireless terminal 608 proactivelyswitches to carrier frequency B once it detects that the current servingcarrier, carrier frequency A, is becoming a boundary carrier. At thetime of handoff, the quality of the current serving carrier is betterthan in traditional handoff scenarios. This results in an improved levelof communications, over traditional handoff scenarios, with minimal orno disruptions in service between wireless terminal 608 and base station602 during the handoff process. Subsequent to the handoff, the multiplecarrier diversity, of the present invention, results in an improvedlevel of performance, over tradition handoffs, because the newoperational carrier, carrier B is not a boundary carrier.

Throughout the area of coverage by the base station 602, the actualpower reception levels of the wireless terminals may vary normally dueto natural condition, e.g. obstructions, weather conditions, etc.Typical multi carrier implementations use only one power level for allthe carrier frequencies at the same base station; however, the presentinvention uses different power levels for different carrier frequenciesat the same base station. If a wireless terminal is operating on afrequency and begins to lose signal due to a natural cause, with thetypical implementation the signal may be expected to have degradedequally on all the potential frequencies, and communications may belost. However; with the implementation of the present invention, thewireless terminal 608 may select to perform an inter-carrier handoff atbase station 602 to another carrier frequency, if available, that hasbeen allocated a higher level of power transmission by the base station602 resulting maintained communications between base station 602 andwireless terminal 608.

FIG. 7 illustrates an exemplary communications system 700 implementingmultiple carrier diversity across both cells and sectors in accordancewith the present invention. The communications system 700 includes aplurality of base stations, base station 1 702 with a coverage areadefined by cell 1 701, base station M 702′ with a coverage area definedby cell N 703. Each base station 702, 702′ of exemplary system 700, asshown, may operate on two carrier frequencies A and B at different powerlevels, in accordance with the invention. For base station 1 702, thepower level for carrier frequency A is less than the power level forcarrier frequency B; therefore, a cell 1 boundary for frequency A (solidline circle) 714 is smaller than a cell 1 boundary for frequency B(dashed line circle) 712. Cell 1 701 includes a coverage area which isthe composite of the areas defined by boundaries 712 and 714. For basestation M 702′, the reverse is true. The power level for carrierfrequency B is less than the power level for carrier frequency A;therefore, a cell N boundary for frequency B (dashed line circle) 734 issmaller than a cell N boundary for frequency A (solid line circle) 732.Cell N 703 includes a coverage area which is the composite of the areasdefined by boundaries 732 and 734. A carrier frequency A boundary regionfor cells 1 and N is represented by the ascending line shaded area 749;a carrier frequency B boundary region for cells 1 and N is representedby the descending line shaded area 750. The two cell boundary regions749 and 750 do not overlap by design in accordance with the invention.In general, base stations may operate on multiple carrier frequencies atdifferent power levels, in accordance with the invention.

Base station 1 702 may transmit to a plurality of sectors, whichsubdivide the cellular coverage area for cell 1. Base station 1 702 isconfigured with a plurality of multisector antennas, one for eachcarrier frequency used. Information on two sectors: designated sector 1and sector Y are shown in FIG. 7 for simplicity. The base station'santennas are offset sufficiently so that the boundary regions betweensectors do not overlap in accordance with the invention. With respectthe base station 1 702, an area with crosshatched shading 716 representsthe sector 1/sector Y boundary area for carrier frequency A; an areawith small circle shading 718 represents the sector 1/sector Y boundaryarea for carrier frequency B. Area 760 represents sector 1 non-boundaryarea for both carrier frequencies A and B. Area 762 represents sector Ynon-boundary area for frequency A and sector 1 non-boundary area forfrequency B. Area 764 represents sector Y non-boundary area for bothfrequencies A and B. Area 766 represents sector 1 non-sector boundaryarea for frequency A and sector Y non-sector boundary area for frequencyB.

Similarly, the cellular coverage area for base station M 702′ may besubdivided into sector boundary areas: sector 1/sector Y boundary areafor frequency A 746 (horizontal line shading), sector 1/sector Yboundary area for frequency B 748 (vertical line shading), andnon-sector boundary areas: sector 1 frequencies A and B area 770, sector1 frequency B/sector Y frequency A area 772, sector Y frequencies A andB area 774, and sector 1 frequency A/sector Y frequency B area 776.

Base station 1 702 is coupled to a plurality of end nodes (ENs), e.g.wireless terminals such as mobile nodes (MNs), fixed wireless devices,etc., in sector 1: EN(1) 704, EN(X) 706 via wireless links 720, 722,respectively. Similarly in sector Y base station 1 702 is coupled to aplurality of end nodes in sector Y: EN(1′) 708, EN(X′) 710 via wirelesslinks 724, 726, respectively.

Similarly, base station M 702′ is coupled to ENs 704′, 706′, 708′, and710′ via wireless links 720′, 722′, 724′, and 726′, respectively.

The ENs 704, 706, 708, 710, 704′, 706′, 708′, and 710′ may movethroughout the system 700, establish a communication session with a peernode, e.g., another end node, establish communication with the basestations 702, 702′, and exchange data and information directly with thebase stations 702, 702′ over the air. The ENs, e.g. EN(1) 704, inaccordance with the invention, proactively monitor signal strengthsand/or quality for available carrier frequencies, identify any celland/or sector boundary carrier frequencies, determine possible candidatecarriers, and select a carrier to use to minimize any boundary problems.The ENs can also decide to make inter-frequency handoffs betweencarriers at a single base station, and may select or change carriersbased on non-boundary considerations, e.g., traffic loading, in order tooptimize performance.

The base stations 702, 702′ are coupled to a network node 740 vianetwork links 742, 744, respectively. The network node may couple thesystem 700 to other network nodes, e.g. other bases stations, accessrouters, intermediate nodes, home agent nodes, or Authentication,Authorization, Accounting (AAA) server nodes via network link 746.Network links 742, 744, and 746 may be, e.g. fiber optic cables.

Consider that an exemplary EN, for example EN(1) 704, is movingthroughout the area of potential coverage for communications with basestation 1 702. If EN 704 is outside boundary 714, it will not usefrequency A to communicate with BS1 702 because of insufficientreception strength. If EN 704 is located in cellular boundary region749, it is restricted, from using frequency A (a cell boundary carrier)to communicate, but may use frequency B to communicate with BS1 702. IfEN 704 is in boundary region 750, it is restricted from using frequencyB (a cell boundary carrier) to communicate, but may use frequency A tocommunicate with BS M 702′. If EN 704 is in sector boundary region 716,it is restricted from using frequency A (a sector boundary carrier), butmay use frequency B to communicate with BS1 702. If EN1 704, is insector boundary region 718, it is restricted from using frequency B (asector boundary carrier) to communicate with BS1 702, but may usefrequency A provided EN704 is within boundary 714. In the remainingareas of potential BS1 702 coverage there is no restriction, and BS1 702may select from either frequency based on other considerations such astraffic loading.

FIG. 8 illustrates an exemplary base station (BS) 800 implemented inaccordance with the present invention. Exemplary base station 800 may bea more detailed representation of base stations 202, 204 of FIG. 2, 402of FIG. 4, 602 of FIGS. 6, and 702, 702′ of FIG. 7. As shown, theexemplary BS 800 includes a receiver circuit 802, transmitter circuit804, processor 806, e.g, CPU, memory 810 and an I/O network interface808 coupled together by a bus 807. The receiver circuit 802 is coupledto one or more antennas 803, 803′ for receiving signals from end nodes900 (See FIG. 9), e.g., wireless terminals such as mobile nodes. Thetransmitter circuit 804 is coupled to one or more transmitter antennas805, 805′ which can be used to broadcast signals to end nodes 900. Inthe sectorized embodiment, the transmitter circuit 804 may include aplurality of sector transmitter circuits, sector 1 transmitter circuitry840, sector N transmitter circuitry 840′. The receiver circuit 802 andthe transmitter circuit 804 shall be capable of operating on a pluralityof carrier frequencies. In some embodiments, the transmitter 804 shalloperate at different power levels corresponding to different carrierfrequencies in order to create distinct cell boundaries for each carrierfrequency. The receiver circuit 802 may include a de-scrambler circuitand the transmitter circuit 804 may include a scrambler circuit invarious embodiments of the invention. The antennas 803, 803′, 805, 805′may be sectorized antennas in various embodiments. In some embodiments,multiple transmitter antennas 805, 805′ may exist for each of the basestation's 800 carrier frequencies, and each sectorized antenna 805, 805′may be offset by a sufficient amount to prevent or minimize sectorboundary overlap regions. In some embodiments, one pair of sectorizedreceiver/transmitter antennas 803/805, 803′/805′ may exist for each ofthe base station's 800 carrier frequencies; each pair of the sectorizedantennas may be offset to prevent or minimize sector boundary overlapregions. The network I/O interface 808 is used to couple the basestation 800 to one or more network elements, e.g., routers and/or theInternet. In this manner, the base station 800 can serve as acommunications element between end nodes 900 serviced by the basestation 800 and other network elements.

Operation of the base station 800 is controlled by the processor 806under direction of one or more routines 812 stored in the memory 810which control the basic functionality of the base station 800 andimplement the various features and methods of the present invention.Memory 810 includes routines 812 and data/information 814. The routines812 include a communications routine 816, signal generation/receptionroutines 818, a scheduler 820, a power management routine 822, a sectormanagement routine 824, and an inter-carrier handoff routine 825.Data/Information 814 includes active user information 826, data 828,carrier frequency information 830, and system parameter information 832.

Communications routines 816, include various communications applicationswhich may be used to provide particular services, e.g., IP telephonyservices or interactive gaming, to one or more end node 900 users.Signal generation/reception routines 818 utilize the data/info 814, e.g,data 828, system parameter information 832, and carrier information 830to provide the required signal synchronization, generation, receptionand processing. The scheduler 820 may perform assignments of users (endnodes 900) to operate: on specific carrier frequencies, on specificchannels using specific sub-carrier frequencies, within specificsectors, at specific times. The scheduler 820 may use active user info826 in making scheduling decisions between various end nodes 900 inorder to minimize disruptions on cell/sector boundaries, moreefficiently load balance the system, and satisfy the needs and requestsof the various end nodes 900. Power management routine 822 may utilizethe data/info 814, e.g., carrier frequency information 830 and systemparameter information 832 to control and regulate the different powerlevels that may be assigned to each carrier frequency used by the basestation 800 thus creating different cell boundaries for differentcarrier frequencies in accordance with one embodiment of the presentinvention. Sector management routine 824 may use the data/info 814,e.g., carrier frequency information 830 to establish and controldifferent non-overlapping sector boundaries for different carrierfrequencies in accordance with one embodiment of the present invention.Inter-carrier handoff routine 825 may utilize the data/info 814including carrier frequency info 830 and active user information 826 toperform a hand-off operation between different carrier frequencies for auser, e.g. mobile node 900, while still maintaining attachment to thesame base station 800, due to a request from a user triggered by itemssuch as: the identification of a sector boundary carrier, theidentification of a cell boundary carrier, a change in conditions, anattempt to better load balance the system in accordance with someembodiments of the invention. In accordance with other embodiments ofthe invention, the decision to perform an inter-carrier hand-offoperation may be made by the base station 800 based on data/info 814,e.g, active user information 826, carrier information 830, e.g., currenttraffic loading on each carrier, and other information available to thebase station 800. In some embodiments, the inter-carrier hand-offroutine 825 may use feed back information from the wireless terminal900, e.g., active user info 826 such as intercell and/or intracellinterference information to determine whether the wireless terminal 900is in a boundary or non-boundary region. In a boundary region intra-celland intra-sector carrier handoffs may be initiated even thoughcommunication with a wireless terminal remains possible using a carrieralready being used to communicate with the wireless terminal. If in anon-boundary region, inter-carrier hand-off routine 825 may makedecisions and perform carrier handoff operations as a function of othersystem consideration such as loading. In some cases, system loading asopposed to interference considerations will trigger an intra-cell and/orintra sector carrier handoff.

Active user information 826 includes information for each active userand/or end node 900 serviced by the base station 800. For each of aplurality of end nodes 900 and/or users it includes a set ofinformation: user 1 info 834, user N info 834′. The user information834, 834′ includes, state information, e.g., whether the mobile node 900is in an on state, a hold state, a sleep state, or an access state,number and types of data packets currently available for transmission toor from the end node 900, assigned carrier frequency, assigned sector,and information on the communication resources used by the end node 900.The user information 834, 834′ may also include information feed backfrom the end node 900 such as received pilot signal strength, recognizedboundary carriers, requested carrier hand-offs, channel qualityinformation, intercell interference information, and intracellinterference information. Data 828 may include data to be transmittedto, or received from, one or more end nodes 900. Examples of data 828may include, e.g., voice data, E-mail messages, video images, game data,etc. Carrier frequency information 830 may include the carrierfrequencies assigned to the base station 800 and associated informationsuch as corresponding power level assignments, actual operational powerlevels, traffic loading, corresponding sector assignments, sectortraffic loading for each carrier frequency and sector specificparameters such as e.g. antenna offsets and sector specificencoding/decoding sequences, and corresponding programmable filtervalues required to process the various carrier frequencies. Systemparameter information 832 may include, e.g., transmitted pilot powerlevels, data/control and pilot hopping sequence values for the cell andsectors of the base station 800.

FIG. 9 illustrates an exemplary end node 900 implemented in accordancewith the invention. Exemplary end node 900 may be a wireless terminal,e.g., mobile node or stationary wireless communications device. End node900 may be a more detailed representation of the wireless terminalpreviously described with respect to the invention in FIGS. 2-8 such asEN 608 of FIG. 6 or EN(1) 704 of FIG. 7. The exemplary end node 900includes a receiver 902 coupled to an antenna 903, a transmitter 904,coupled to an antenna 905, a memory 910 and a processor 906. Thereceiver 902, in the illustrated embodiment includes a single receiverchain including a channel filter 951 and demodulator 952. In someembodiments, multiple receiver chains are used to allow for multiplecarriers to be received and processed at the same time. Channel filter951 is adjustable so that the passband of the filter can be selected tocorrespond to the carrier being received at any point in time. Thevarious elements of the end node 900: receiver 902, transmitter 904,processor 906, and memory 910 are coupled together via bus 907. The endnode 900 uses its transmitter 904, receiver 902, and antennas 905, 903to send and receive information to and from base station 800. Thetransmitter 904 and receiver 902 shall be capable of operating onmultiple carrier frequencies as utilized by the various base stations800. In some embodiments, the transmitter 904 and the receiver 902 mayinclude encoder/decoder circuits to match the base stations 800. Invarious embodiments of the invention, the receiver 902 and/or thetransmitter 904 shall have programmable analog filters to allow a singleanalog circuit path to be utilized for multiple carrier frequencies thusreducing cost. Various embodiments of the receiver 902 are possible aspreviously described with respect to FIG. 3 including: two RF andbaseband receiver chains where one chain is tuned to a particularcarrier and the other chain searches for alternative carriers, one RFand baseband receiver chain where the receiver 902 uses the times duringwhich no reception is required from the current serving carrier tomonitor other alternative carriers, and a single RF receiver capable ofreceiving signals encompassing several carriers and have basebandreceiver which can tune to different carriers and measure their signalstrength.

Memory 910 includes routines 912 and data/info 914. The routines 912 mayinclude a communications routine 916, signal generation/receptionroutines 918, a transmission power control and power control signalingroutine 950 including a carrier strength measurement routine 920, acell/sector boundary identification routine 922, and a carrier selectionroutine 924. The data/info 914 may include user/device information 926,data 928, carrier information 930, and system parameter information 932.The end node 900 operates under control of the modules or routines 912,which are executed by the processor 906 in order to perform the basicfunctionality of the end node 900 and implement the methods andimprovements in accordance with the present invention. User/deviceinformation 926 includes device information, e.g., a device identifier,a network address or a telephone number. This information can be used,by the base station 800, to identify the end nodes 900, e.g., whenassigning communications channels. The user/device information 926includes information concerning the present state of the end node 900,e.g., whether the mobile node 900 is in an on state, a hold state, asleep state, or an access state, number and types of data packetscurrently available for transmission to or from the base station 800,levels of overall interference, intercell interference for each carrier,intersector interference for each carrier. The data 928 includes, e.g.,voice, text and/or other data received from, or to be transmitted to,the base station 800 as part of a communications session. Carrierinformation 930 may include information such as carrier measured pilotstrength levels for detected carrier frequencies, active list ofcandidate carriers, intercell channel interference, intersector channelinterference, active list of identified cell/sector boundary carriers,active carrier, requested new carrier, etc. System parameter information932 may include information such as carrier frequency assignments tospecific cells/base stations and/or sectors, hopping sequenceparameters, coding sequences used, classifications of types ofinterference, and criteria levels used for classification of a carrieras a cell/sector boundary carrier, and criteria used for initiating anintercarrier handoff.

Communications routines 916, include various communications applicationswhich may be used to provide particular services, e.g., IP telephonyservices or interactive gaming, to one or more end node users. Signalgeneration/reception routines 918 utilize the data/info 914, e.g, data928, system parameter information 932 such as hopping sequence values,user device info 926 such as device ID, carrier information 930 such asthe current active carrier to provide the required signal timingcontrol, synchronization, and signal generation and signal reception.The signal generation/reception routine 918 controls the transmissionand the reception of payload data, e.g., a channel or time slotdedicated to the end node 900 for signaling purposes. Routine 918 mayalso control the operation of receiver 902 and the transmitter 904including the setting of the programmable analog filters to the selectedcarrier frequencies.

Transmission power control and power control signaling routine 950 isused to control the generation, processing and reception of transmissionpower control signals. Module 950 controls the signaling used toimplement transmission power control through interaction with the basestation 800. Signals transmitted to, or received from the base station800 are used to control end node 900 transmission power levels underdirection of module 950. Power control is used by the base station 800and the end nodes 900 to regulate power output when transmittingsignals. The base station 800 transmits signals to the end nodes 900which are used by the end nodes 900 in adjusting their transmissionpower output. The optimal level of power used to transmit signals varieswith several factors including transmission burst rate, channelconditions and distance from the base station 800, e.g., the closer theend node 900 is to the base station 800, the less power the mobile node900 needs to use to transmit signals to the base station 800. Using amaximum power output for all transmissions has disadvantages, e.g., theend node 900 battery life is reduced, and high power output increasesthe potential of the transmitted signals causing interference, e.g.,with transmissions in neighboring or overlapping cells and or sectors.Transmission power control signaling allows the end node 900 to reduceand/or minimize transmission output power and thereby extend batterylife.

Carrier signal strength measuring routine 920, included in power routine950, monitors the signal strengths, e.g. pilots, and/or quality for allthe carriers received by the end node 900 periodically and/orrepetitively and stores the information as part of the carrierinformation 930 to be used by the cell/sector boundary identificationroutine 922 and the carrier selection routine 924 in accordance with theinvention. Routine 920 may use the user/device info 926, e.g. state, todetermine when to switch the receiver 902 to search for alternativecarriers. Routine 920 may also control the switching within the receiver902 between different programmable filters values for different carrierfrequencies as the receiver 902 searches for all carriers. Inaccordance, with the invention, the carrier signal strength monitoringroutine 920 is performed by end node 900 in a proactive manner; thisallows transitions between carriers as the level of interferenceincreases or the signal strength begins to degrade allowing the end node900 to transition to a new carrier with minimal or no disruption incommunications.

The cell boundary/sector boundary identification routine 922 identifiescell and sector boundary carriers utilizing, e.g., the carrier strengthmeasurement information collected and applying rejection criteriadefined in the system parameter info 932. The cell/sector boundaryidentification routine 922 estimates intercell channel interference dueto transmissions from the base stations. The cell/sector boundaryidentification routine 922 estimates intersector channel interferencedue to transmissions from various sector of the same base station.Routine 924 generates a list of candidate carriers (part of carrier info930) by applying an acceptable criteria, e.g., a signal strengthcriteria (part of system parameter info 932), to the list of measuredcarriers excluding identified cell/sector boundary carriers.

Carrier signal strength routine 920 and cell/boundary identificationroutine 922 may allow wireless terminal 900 to distinguish betweenintracell interference and other types of interference. Routines 920 and922 may also allow wireless terminal 900 to distinguish betweenintersector interference and other types of interference.

Carrier selection routine 924 utilizes the list of candidate carriers toselect a carrier for the end node 900 to use. The carrier selectionroutine 924 may apply additional criteria, such as system trafficloading, power considerations, anticipated entry into a sector/cellboundary region to select which carrier to use.

The present invention may be implemented in hardware, software, or acombination of hardware and software. For example, some aspects of theinvention may be implemented as processor executed program instructions.Alternately, or in addition, some aspects of the present invention maybe implemented as integrated circuits, such as, for example, ASICs.

Numerous additional variations on the methods and apparatus of thepresent invention described above will be apparent to those skilled inthe art in view of the above descriptions of the invention. Suchvariations are to be considered within the scope of the invention.

What is claimed is:
 1. A method of a wireless terminal, comprising:communicating with a base station using a first carrier provided withina first coverage area; determining that the wireless terminal is in aboundary region for the first carrier, the first carrier is subject tointer-cell interference or inter-sector interference greater than athreshold, and that the first carrier is a boundary carrier; determiningat least one candidate carrier provided by the base station that is anon-boundary carrier, the at least one candidate carrier excluding thefirst carrier and including a second carrier provided within a secondcoverage area; and initiating a handoff from the first carrier to thesecond carrier.
 2. The method of claim 1, wherein the first carrier issubject to inter-cell interference greater than a threshold, theinter-cell interference is due to the first carrier being utilized by aneighboring cell of a neighboring base station, the first carrier isprovided by the base station at a first power and the second carrier isprovided by the base station at a second power greater than the firstpower, the first coverage area is within the second coverage area, thecommunicating and the initiating occur within the first coverage area,and the handoff is an intra-cell handoff to the second carrier providedby the base station.
 3. The method of claim 1, wherein the first carrieris subject to inter-cell interference greater than a threshold, theinter-cell interference is due to the first carrier being utilized by aneighboring cell of a neighboring base station, the first carrier isprovided by the base station at a first power and the second carrier isprovided by the base station at a second power less than the firstpower, the second coverage area is within the first coverage area, thecommunicating and the initiating occur within the first coverage areaand outside the second coverage area, and the handoff is an inter-cellhandoff to the second carrier provided by the neighboring base station.4. The method of claim 1, wherein the first carrier is subject tointer-sector interference greater than the threshold, the inter-sectorinterference is due to the first carrier being utilized by anoverlapping cell of the base station, the first carrier is provided bythe base station at a first power and the second carrier is provided bythe base station at a second power greater than the first power, thefirst coverage area is within the second coverage area, thecommunicating and the initiating occur within the first coverage area,and the handoff is an intra-cell handoff to the second carrier providedby the base station.
 5. The method of claim 1, wherein the first carrieris subject to inter-sector interference greater than the threshold, theinter-sector interference is due to the first carrier being utilized byan overlapping cell of the base station, the first carrier is providedby the base station at a first power and the second carrier is providedby the base station at a second power less than the first power, thesecond coverage area is within the first coverage area, thecommunicating and the initiating occur within the second coverage area,and the handoff is an intra-cell handoff to the second carrier providedby the base station.
 6. A wireless terminal, comprising: means forcommunicating with a base station using a first carrier provided withina first coverage area; means for determining that the wireless terminalis in a boundary region for the first carrier, the first carrier issubject to inter-cell interference or inter-sector interference greaterthan a threshold, and that the first carrier is a boundary carrier;means for determining at least one candidate carrier provided by thebase station that is a non-boundary carrier, the at least one candidatecarrier excluding the first carrier and including a second carrierprovided within a second coverage area; and means for initiating ahandoff from the first carrier to the second carrier.
 7. The wirelessterminal of claim 6, wherein the first carrier is subject to inter-cellinterference greater than a threshold, the inter-cell interference isdue to the first carrier being utilized by a neighboring cell of aneighboring base station, the first carrier is provided by the basestation at a first power and the second carrier is provided by the basestation at a second power greater than the first power, the firstcoverage area is within the second coverage area, the means forcommunicating communicates with the base station and the means forinitiating initiates the handoff within the first coverage area, and thehandoff is an intra-cell handoff to the second carrier provided by thebase station.
 8. The wireless terminal of claim 6, wherein the firstcarrier is subject to inter-cell interference greater than a threshold,the inter-cell interference is due to the first carrier being utilizedby a neighboring cell of a neighboring base station, the first carrieris provided by the base station at a first power and the second carrieris provided by the base station at a second power less than the firstpower, the second coverage area is within the first coverage area, themeans for communicating communicates with the base station and the meansfor initiating initiates the handoff within the first coverage area andoutside the second coverage area, and the handoff is an inter-cellhandoff to the second carrier provided by the neighboring base station.9. The wireless terminal of claim 6, wherein the first carrier issubject to inter-sector interference greater than the threshold, theinter-sector interference is due to the first carrier being utilized byan overlapping cell of the base station, the first carrier is providedby the base station at a first power and the second carrier is providedby the base station at a second power greater than the first power, thefirst coverage area is within the second coverage area, the means forcommunicating communicates with the base station and the means forinitiating initiates the handoff within the first coverage area, and thehandoff is an intra-cell handoff to the second carrier provided by thebase station.
 10. The wireless terminal of claim 6, wherein the firstcarrier is subject to inter-sector interference greater than thethreshold, the inter-sector interference is due to the first carrierbeing utilized by an overlapping cell of the base station, the firstcarrier is provided by the base station at a first power and the secondcarrier is provided by the base station at a second power less than thefirst power, the second coverage area is within the first coverage area,the means for communicating communicates with the base station and themeans for initiating initiates the handoff within the second coveragearea, and the handoff is an intra-cell handoff to the second carrierprovided by the base station.
 11. A wireless terminal, comprising: aprocessor configured to: communicate with a base station using a firstcarrier provided within a first coverage area; determine that thewireless terminal is in a boundary region for the first carrier, thefirst carrier is subject to inter-cell interference or inter-sectorinterference greater than a threshold, and that the first carrier is aboundary carrier; determine at least one candidate carrier provided bythe base station that is a non-boundary carrier, the at least onecandidate carrier excluding the first carrier and including a secondcarrier provided within a second coverage area; and initiate a handofffrom the first carrier to the second carrier.
 12. The wireless terminalof claim 11, wherein the first carrier is subject to inter-cellinterference greater than a threshold, the inter-cell interference isdue to the first carrier being utilized by a neighboring cell of aneighboring base station, the first carrier is provided by the basestation at a first power and the second carrier is provided by the basestation at a second power greater than the first power, the firstcoverage area is within the second coverage area, the processor isconfigured to communicate with the base station and to initiate thehandoff within the first coverage area, and the handoff is an intra-cellhandoff to the second carrier provided by the base station.
 13. Thewireless terminal of claim 11, wherein the first carrier is subject tointer-cell interference greater than a threshold, the inter-cellinterference is due to the first carrier being utilized by a neighboringcell of a neighboring base station, the first carrier is provided by thebase station at a first power and the second carrier is provided by thebase station at a second power less than the first power, the secondcoverage area is within the first coverage area, the processor isconfigured to communicate with the base station and to initiate thehandoff within the first coverage area and outside the second coveragearea, and the handoff is an inter-cell handoff to the second carrierprovided by the neighboring base station.
 14. The wireless terminal ofclaim 11, wherein the first carrier is subject to inter-sectorinterference greater than the threshold, the inter-sector interferenceis due to the first carrier being utilized by an overlapping cell of thebase station, the first carrier is provided by the base station at afirst power and the second carrier is provided by the base station at asecond power greater than the first power, the first coverage area iswithin the second coverage area, the processor is configured tocommunicate with the base station and to initiate the handoff within thefirst coverage area, and the handoff is an intra-cell handoff to thesecond carrier provided by the base station.
 15. The wireless terminalof claim 11, wherein the first carrier is subject to inter-sectorinterference greater than the threshold, the inter-sector interferenceis due to the first carrier being utilized by an overlapping cell of thebase station, the first carrier is provided by the base station at afirst power and the second carrier is provided by the base station at asecond power less than the first power, the second coverage area iswithin the first coverage area, the processor is configured tocommunicate with the base station and to initiate the handoff within thesecond coverage area, and the handoff is an intra-cell handoff to thesecond carrier provided by the base station.
 16. A computer programproduct in a wireless terminal, comprising: a computer-readable mediumcomprising code for: communicating with a base station using a firstcarrier provided within a first coverage area; determining that thewireless terminal is in a boundary region for the first carrier, thefirst carrier is subject to inter-cell interference or inter-sectorinterference greater than a threshold, and that the first carrier is aboundary carrier; determining at least one candidate carrier provided bythe base station that is a non-boundary carrier, the at least onecandidate carrier excluding the first carrier and including a secondcarrier provided within a second coverage area; and initiating a handofffrom the first carrier to the second carrier.
 17. The computer programproduct of claim 16, wherein the first carrier is subject to inter-cellinterference greater than a threshold, the inter-cell interference isdue to the first carrier being utilized by a neighboring cell of aneighboring base station, the first carrier is provided by the basestation at a first power and the second carrier is provided by the basestation at a second power greater than the first power, the firstcoverage area is within the second coverage area, the code forcommunicating communicates with the base station and the code forinitiating initiates the handoff within the first coverage area, and thehandoff is an intra-cell handoff to the second carrier provided by thebase station.
 18. The computer program product of claim 16, wherein thefirst carrier is subject to inter-cell interference greater than athreshold, the inter-cell interference is due to the first carrier beingutilized by a neighboring cell of a neighboring base station, the firstcarrier is provided by the base station at a first power and the secondcarrier is provided by the base station at a second power less than thefirst power, the second coverage area is within the first coverage area,the code for communicating communicates with the base station and thecode for initiating initiates the handoff within the first coverage areaand outside the second coverage area, and the handoff is an inter-cellhandoff to the second carrier provided by the neighboring base station.19. The computer program product of claim 16, wherein the first carrieris subject to inter-sector interference greater than the threshold, theinter-sector interference is due to the first carrier being utilized byan overlapping cell of the base station, the first carrier is providedby the base station at a first power and the second carrier is providedby the base station at a second power greater than the first power, thefirst coverage area is within the second coverage area, the code forcommunicating communicates with the base station and the code forinitiating initiates the handoff within the first coverage area, and thehandoff is an intra-cell handoff to the second carrier provided by thebase station.
 20. The computer program product of claim 16, wherein thefirst carrier is subject to inter-sector interference greater than thethreshold, the inter-sector interference is due to the first carrierbeing utilized by an overlapping cell of the base station, the firstcarrier is provided by the base station at a first power and the secondcarrier is provided by the base station at a second power less than thefirst power, the second coverage area is within the first coverage area,the code for communicating communicates with the base station and thecode for initiating initiates the handoff within the second coveragearea, and the handoff is an intra-cell handoff to the second carrierprovided by the base station.